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Optimizing Thermal Processes

Using Advanced Rotary Tube Internals to Improve Thermal Processes

| Author / Editor: Mike Maggio / Wolfgang Ernhofer

There are many arguments for the use of advanced internals. One of the best are better efficiency and product quality.
There are many arguments for the use of advanced internals. One of the best are better efficiency and product quality. (Picture: Harper)

The use of tube internals in a rotary furnace can increase heat transfer surface area, improve radial mixing, improve gas-solid interactions, control gas flow direction, and even control material dwell time or velocity through the tube. All of these can result in a reduction of residence time required or increase unit capacity for a given volume of furnace but can also improve product quality and control of quality and uniformity.

Thermal process of powders – including calcination, sintering, carburization, etc. – is dominantly performed, due to its inherent characteristics, in rotary tube furnace. Rotary furnaces are known to be the preferred processing unit for flowable powdered or granular materials. The rotary tube furnace, under the proper conditions, will be significantly more energy efficient than the pusher tunnel furnace.

The energy requirement of heating the rigid furniture of the pusher tunnel furnace is replaced by the stationary but rotating tube of the rotary, which sets the rotary apart with significant lower total energy consumption. But what is less widely known about the rotary, and what will be discussed, is a rotary’s capability of becoming even more energy efficient with the use of tube internals.

Surface Area – The Elementary Factor for Heat Transfer

At its most elementary function, a tube internal for a rotary furnace will increase the surface area for heat transfer. Through its mere presence in the heating section of the rotary furnace the internals will be maintained at temperature (a lower temperature than the tube wall but higher than the process material, thermal conductivity of steal vs. bulk material) and in turn provide another surface, other than the inner wall of the process tube, to transfer heat to the product. This increased heat transfer area can be thought of as the fins or plates of a heat exchanger; an increase in the fins or plates of a heat exchanger will increase the total heat transfer of the exchanger.

Improving the Surface Area helps to Optimize Thermal Processes

When considered in the preliminary designs of the rotary furnace, the added heat transfer surface area can reduce the required total processing length of the rotary furnace. If the intention is to add internals to an existing rotary furnace it is conceivable that the added heat transfer surface area may reduce the required residence time for processing which will in turn increase the mass throughput capabilities of the unit.

Increasing more surface area does not just aid in the heating of the process material but can also aid in cooling the product post processing. Many rotary tube cooling chamber do not contain internals in the overall design, they are typically a simple water cooled jacket around the outside of the rotary tube, but this is not always efficient. By introducing more surface area inside the tube, the material with see more cold surfaces and cool more efficiently.

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